Graded-index optical fiber lenses (GRIN fiber lenses) are important components in optical fiber communication systems. A GRIN fiber lens comprises, in essence, a short length of optical fiber having a refractive index which is graded to act as a lens. Grading in this context means that the index of refraction of the fiber varies as a function of the radial distance from the center of the fiber. Transmitted light tends to bend toward regions of greater refractive index, so the graded-index fiber acts as a lens.
GRIN fiber lenses are highly useful for collimating the output of an optical fiber and for changing the spot diameter of an output beam. Collimating the fiber output permits the output signal to travel long distances outside the fiber without excessive diffraction. Increasing the beam spot size reduces the power density of the emerging beam and thus reduces the deleterious effects of high power. Changing the spot size also facilitates coupling a fiber to another fiber or device of different spot size. GRIN fiber lenses are particularly advantageous for such applications because they can be drawn to the same diameter as a transmission fiber and be fused to the transmission fiber by a low loss junction. The structure and applications of GRIN fiber lenses are described in greater detail in U.S. Pat. No. 4,701,011 issued to Emkey et al. on Oct. 20, 1987. The '011 patent is incorporated herein by reference.
Unfortunately it is very difficult to fabricate GRIN fiber lenses having precisely defined lens characteristics. GRIN lenses are typically fabricated via the modified chemical vapor deposition (MCVD) process by first forming a graded tubular optical preform and then drawing fiber from the preform. The preforms are commercially made using thermal chemical vapor reactions to deposit mixed oxides as layers of glass soot (particles) on the inside surface of a glass starting tube. The proportions of the reactants are varied as the thickness increases within the tube to grade the refractive index. Upon completion of the deposition, the tube is collapsed into a solid rod preform, and fiber is drawn from the preform in the conventional manner. The drawn fiber has a refractive index profile similar to that of the preform.
A primary problem in the fabrication of GRIN lenses is the difficulty of controlling the refractive index profile of the GRIN lens fiber by controlling the index profile of the preform. The desired refractive index profile of the GRIN fiber lens depends upon the application and the required optical performance. But changing the fiber profile by changing the composition profile of deposited glass is particularly difficult. The deposition equipment must be tuned in a tedious, time-consuming and expensive process. Accordingly there is a need for an improved method for fabricating GRIN fiber lenses with increased control over the refractive index profile.